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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Semantics</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="C++, library, debug" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="debug_mode.html" title="Chapter 17. Debug Mode" /><link rel="prev" href="debug_mode.html" title="Chapter 17. Debug Mode" /><link rel="next" href="debug_mode_using.html" title="Using" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Semantics</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="debug_mode.html">Prev</a> </td><th width="60%" align="center">Chapter 17. Debug Mode</th><td width="20%" align="right"> <a accesskey="n" href="debug_mode_using.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.ext.debug_mode.semantics"></a>Semantics</h2></div></div></div><p>
  </p><p>A program that uses the C++ standard library correctly
  will maintain the same semantics under debug mode as it had with
  the normal (release) library. All functional and exception-handling
  guarantees made by the normal library also hold for the debug mode
  library, with one exception: performance guarantees made by the
  normal library may not hold in the debug mode library. For
  instance, erasing an element in a <code class="code">std::list</code> is a
  constant-time operation in normal library, but in debug mode it is
  linear in the number of iterators that reference that particular
  list. So while your (correct) program won't change its results, it
  is likely to execute more slowly.</p><p>libstdc++ includes many extensions to the C++ standard library. In
  some cases the extensions are obvious, such as the hashed
  associative containers, whereas other extensions give predictable
  results to behavior that would otherwise be undefined, such as
  throwing an exception when a <code class="code">std::basic_string</code> is
  constructed from a NULL character pointer. This latter category also
  includes implementation-defined and unspecified semantics, such as
  the growth rate of a vector. Use of these extensions is not
  considered incorrect, so code that relies on them will not be
  rejected by debug mode. However, use of these extensions may affect
  the portability of code to other implementations of the C++ standard
  library, and is therefore somewhat hazardous. For this reason, the
  libstdc++ debug mode offers a "pedantic" mode (similar to
  GCC's <code class="code">-pedantic</code> compiler flag) that attempts to emulate
  the semantics guaranteed by the C++ standard. For
  instance, constructing a <code class="code">std::basic_string</code> with a NULL
  character pointer would result in an exception under normal mode or
  non-pedantic debug mode (this is a libstdc++ extension), whereas
  under pedantic debug mode libstdc++ would signal an error. To enable
  the pedantic debug mode, compile your program with
  both <code class="code">-D_GLIBCXX_DEBUG</code>
  and <code class="code">-D_GLIBCXX_DEBUG_PEDANTIC</code> .
  (N.B. In GCC 3.4.x and 4.0.0, due to a bug,
  <code class="code">-D_GLIBXX_DEBUG_PEDANTIC</code> was also needed. The problem has
  been fixed in GCC 4.0.1 and later versions.) </p><p>The following library components provide extra debugging
  capabilities in debug mode:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><code class="code">std::array</code> (no safe iterators)</p></li><li class="listitem"><p><code class="code">std::basic_string</code> (no safe iterators and see note below)</p></li><li class="listitem"><p><code class="code">std::bitset</code></p></li><li class="listitem"><p><code class="code">std::deque</code></p></li><li class="listitem"><p><code class="code">std::list</code></p></li><li class="listitem"><p><code class="code">std::map</code></p></li><li class="listitem"><p><code class="code">std::multimap</code></p></li><li class="listitem"><p><code class="code">std::multiset</code></p></li><li class="listitem"><p><code class="code">std::set</code></p></li><li class="listitem"><p><code class="code">std::vector</code></p></li><li class="listitem"><p><code class="code">std::unordered_map</code></p></li><li class="listitem"><p><code class="code">std::unordered_multimap</code></p></li><li class="listitem"><p><code class="code">std::unordered_set</code></p></li><li class="listitem"><p><code class="code">std::unordered_multiset</code></p></li></ul></div><p>N.B. although there are precondition checks for some string operations,
e.g.  <code class="code">operator[]</code>,
they will not always be run when using the <code class="code">char</code> and
<code class="code">wchar_t</code> specializations (<code class="code">std::string</code> and
<code class="code">std::wstring</code>).  This is because libstdc++ uses GCC's
<code class="code">extern template</code> extension to provide explicit instantiations
of <code class="code">std::string</code> and <code class="code">std::wstring</code>, and those
explicit instantiations don't include the debug-mode checks.  If the
containing functions are inlined then the checks will run, so compiling
with <code class="code">-O1</code> might be enough to enable them.  Alternatively
<code class="code">-D_GLIBCXX_EXTERN_TEMPLATE=0</code> will suppress the declarations
of the explicit instantiations and cause the functions to be instantiated
with the debug-mode checks included, but this is unsupported and not
guaranteed to work.  For full debug-mode support you can use the
<code class="code">__gnu_debug::basic_string</code> debugging container directly,
which always works correctly.
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